This invention relates to pulse discriminators and, more particularly, to a pulse detector and/or filter capable of detecting pulses having widths greater than a first predetermined interval and less than a second predetermined interval while filtering pulse signals having widths less than the second predetermined interval.
There are numerous applications in which it is desirable to detect and/or filter pulse signals depending on a prescribed characteristic of the pulse, for example, pulse width. It has been desirable to detect pulse signals which have pulse widths greater than a first predetermined interval but less than a second predetermined interval. In specific applications it is also desirable not to detect but to filter pulse signals which have widths less than the first predetermined interval and to detect and filter pulse signals which fall within the time interval window between the first and second predetermined intervals, while passing undetected those signals which have widths greater than the second predetermined interval.
Such detectors and filters have found use in telephone supervisory and signaling systems. In particular, they have been employed in so-called "wink" signaling systems. A wink signal is a momentary change in the supervisory condition present on a telephone wire signaling lead that is present for more than a first predetermined interval but less than a second predetermined interval. That is to say, a valid wink signal is a pulse signal having a pulse width that falls within a prescribed time interval window. These wink signals are employed in various telephone systems for different purposes. For example, in one system a wink signal is employed to alert the switching system that multifrequency tone signals are to follow so that proper equipment may be made operative or otherwise attached to the incoming transmission path in order to receive the multifrequency tones. However, since wink signals are employed for other purposes in other systems and equipments, it is also necessary that the wink signal not be propagated beyond the detection equipment in a particular office. Thus, the need for filtering wink signals. It is also important that other signals on the signaling lead be propagated without being filtered, for example, on-hook and off-hook supervisory signals.
Prior arrangements which have been employed to achieve the desired functions used complex time subtraction and/or complex time delay circuits. One prior arrangement employs a plurality of monostable multivibrators and logic gates to realize a pulse width filtering function which allows only pulses within a prescribed time window to pass. These prior circuits, although found to be satisfactory in some applications, are unsatisfactory in others because of their complexity, cost and/or space usage.